In a boiling water reactor, a jet pump system is adopted to increase power density. The jet pump system forcibly circulates reactor coolant as cooling water and includes an external recirculating system and an internal recirculating system as systems for forcibly circulating reactor coolant through a core portion of a reactor pressure vessel.
The external recirculating system includes a plurality of jet pumps in a reactor pressure vessel and a recirculating pump outside the reactor pressure vessel. Cooling water fed from the recirculating pump is jetted by the jet pumps and reactor water around the jet pumps is drawn and forcibly fed into a core portion from a core bottom plenum disposed under the core portion, so that the reactor coolant is forcibly recirculated in the reactor pressure vessel.
FIG. 1 is a vertical cross-sectional view schematically showing a configuration of a boiling water reactor in which a jet pump system of the external recirculating system is adopted. A reactor pressure vessel 1 contains reactor coolant 2 and a core 3. The core 3 includes a plurality of fuel assemblies and control rods, not shown, and is housed in a core shroud 10.
The reactor coolant 2 passes through the core 3 upward and is simultaneously heated by nuclear reaction heat of the core 3 and then becomes a two-phase flow of water and steam. The coolant 2 in the two-phase state flow into a steam separator 4 installed above the core 3 and is separated into water and steam. The steam is introduced into a steam dryer 5 above the steam separator 4 to obtain dry steam, and the dry steam is transferred into a steam turbine, not shown, through a main steam line 6 and is used for power generation. A downcomer 7 between the core shroud 10 and the reactor pressure vessel 1 contains a plurality of jet pumps 11 spaced at regular intervals in a circumferential direction. The water separated by the steam separator 4 is pressurized through a recirculation system, not shown, is introduced into the jet pumps 11 from recirculation inlet nozzles 13, and flows under the core 3 through the jet pumps 11.
FIG. 2 is an enlarged perspective view showing a principle part of the jet pump 11 of FIG. 1. As shown in FIG. 2, the jet pump 11 includes a vertical riser tube 12 that introduces the coolant 2, which has been supplied from the recirculation inlet nozzle 13 of a recirculating pump, not shown, as an upward flow inside the reactor. The upper part of the riser tube 12 is connected to a pair of elbows 15 via a transition piece 14. The elbows 15 split the coolant into two downward flows. The elbows 15 are each connected to an inlet throat 17 via a mixing nozzle 16. The mixing nozzle 16 discharges the coolant 2 and surrounding reactor water is drawn with the coolant 2. The discharged coolant 2 and the drawn reactor water are mixed in the inlet throat 17. The inlet throats 17 are each connected to a diffuser 18 that feeds the coolant below the core. The elbow 15, the mixing nozzle 16, and the inlet throat 17 are integrated into a single unit called inlet mixer 51.
In the case of jet pumps constituting a boiling water reactor, unfortunately, crud of iron oxide in the reactor water is deposited and builds up on surfaces of jet pump members constituting the jet pump, which increases a pressure loss and reduces a flow rate, resulting in lower circulation efficiency. The components of the reactor internal structure provides like or similar problem. For example, crud (CRUD: Chalk River Unclassified Deposit) is considerably deposited and builds up on the jet pump members constituting the inlet mixer exposed to a high flow rate of hot water.
This matter has been dealt with at present by increasing the speeds of recirculating pumps (PLR pumps), which however has caused a large energy loss.
Further, although a water jet cleaning method has been also proposed to remove the deposited crud, this involves extremely high cost, thus being not practical.
Moreover, formation of a coating on surfaces of jet pump members has been proposed to suppress deposition of crud on reactor internal structures including the jet pump members. For example, in methods proposed in specifications of Japanese Patent Laid-Open No. 2002-207094 (Patent Document 1) and U.S. Pat. No. 6,633,623 (Patent Document 2), coatings of oxides including TiO2, ZrO2, Ta2O5, and SiO2 are formed on surfaces of the jet pump members by a CVD (chemical vapor deposition) method or process. Further, in methods proposed in specifications of Japanese Patent Laid-Open No. 2007-10668 (Patent Document 3) and U.S. Patent Application Publication No. 2007/0003001 (Patent Document 4), coatings of platinum, rhodium, iridium, palladium, silver, and gold or metal alloys thereof are formed on surfaces of component parts such as jet pump members by methods or means of, e.g., plasma spray coating, HVOF, CVD, PVD, electroplating, and electroless plating.
As mentioned above, in reactor internal structures such as jet pumps of a boiling water reactor, the crud in reactor water is deposited and builds up on, e.g., surfaces of jet pump members constituting the reactor internal structures, which might increase a pressure loss and a flow rate, resulting in lower circulation efficiency. In order to improve this matter, it has been proposed, in a conventional technology, to form coatings on the surfaces of the jet pump members to thereby suppress adhesion of deposited crud such as disclosed in the related art (Patent Documents 1 to 4). In these proposals, however, deposition of crud cannot be sufficiently suppressed by forming the coatings. Moreover, the formation of the coatings requires an expensive apparatus, and size and shape of members to be coated are limited.